(364) Forward weight prediction among SGA, AGA, and LGA neonates

Objective: The Gestation-Adjusted Projection (GAP) forward projection model extrapolates birth weight at delivery from third-trimester growth ultrasounds. This method relies on the observation that the ratio of the weight of the normal fetus to the population median remains constant in the third trimester. This study aims to evaluate weight prediction accuracy using GAP forward projection from 3^rd trimester growth assessments from neonates ultimately with LGA, SGA, and AGA birthweights at delivery and examine the effect of elevated maternal body habitus on accuracy of these estimates.

Study Design: We conducted a retrospective analysis of pregnancy records from 2016 to 2023 of patients cared for and delivered in our hospital system. We included singleton, liveborn, and non-anomalous pregnancies delivered after 28 weeks GA. Exclusions were multiple gestation, major fetal anomalies, stillbirth, and absence of third trimester growth assessments or mid-gestational anatomic surveys. We identified 1559 records that resulted in review of 554 records with final growth ultrasounds. Accuracy of GAP prediction was defined as an estimation of birth weight within 10% of actual birth weight. Percent error and absolute value of percent error of estimation were also examined.

Results: We observed an overall median absolute value of accuracy of 8.56 (IQR 3.9, 15.19). Across all pregnancies, 57.2% of fetal weight estimates were accurate. Respective groups showed a 51.4% accuracy in women with appropriate BMI (n = 185), 61.4% accuracy in the overweight category (n = 145), and 58% in the obese category (n = 224). Rates of accuracy, overestimation, and underestimation were significantly different between weight classifications (p=0.031).

Conclusion: The GAP forward projection method can be used as an alternative method to estimating fetal weight rather than extrapolation by adding stagnant estimates of grams per day or by using Leopold’s maneuvers which can be challenging on patients of higher maternal BMI.